The invention relates to an electro-optical device for recording and/or reading information in an information plane in an optical record carrier by means of a radiation beam. The device includes a frame carrying a turntable rotatable about an axis of rotation and an optical unit having an objective arranged on a slide pivotable about a pivotal axis which is stationary relative to the frame, to focus the radiation beam so as to form a radiation spot in the information plane. The pivotal axis extends at least substantially transversely of the plane defined by the optical axis and the axis of rotation. The device further includes a slide supporting arrangement and a slide-drive arrangement for moving the objective radially of the turntable between a first position situated near the turntable and a second position which is more remote thereof, and means for tilting the slide.
Such an electro-optical device is known from U.S. Pat. No. 4,637,004 (herewith incorporated by reference) and can be used in optical disc record players.
In electro-optical devices of the type defined above the radiation beam emitted by a laser is concentrated by the objective to form a tiny radiation spot. The radiation spot comprises a bright central portion surrounded by concentric rings of lower intensity. When the center of the radiation spot is situated at an information track the radiation will also be partly incident on adjacent tracks. The radiation which is reflected therefore not only contains the information of the track being scanned but also information pertaining to the adjacent tracks. The reflected radiation is received by radiation sensitive electronic means of the device in order to produce electrical signals corresponding to the information in the reflected radiation. The resulting crosstalk between adjacent information tracks largely depends on the radiation distribution within the radiation spot in the information plane, which radiation distribution may be influenced considerably by an oblique position of the information surface of the disc distribution. An oblique position of the information surface results in an increased radiation intensity towards one side of the central portion, namely in the direction in which the disc surface is inclined, so that during reading, in particular of a disc which is disposed obliquely in a radial direction, the crosstalk between the information track scanned by the radiation spot and the adjacent tracks increases.
An oblique position of an optical disc in a radial direction is mainly caused by out-of-flatness of the disc surface under its own weight. Furthermore, material stresses in the multi-layer optical discs may give these discs a paraboloid shape, also resulting in an oblique position in a radial direction.
The electro-optical device disclosed in U.S. Pat. No. 4,637,004 has correction means which during recording and/or reading of the information tracks provide a correction for the neutral position of the optical axis by tilting the objective about a pivotal axis perpendicular to the optical axis through a correction angle which is related to the direction of the normal to the information surface of the optical disc at the location of the intersection between the surface and the optical axis of the objective. For this purpose the known electro-optical device has a frame, a subframe, and a slide which carries the optical scanning unit and which is translatable relative to the subframe. The slide serves for translating the scanning unit relative to the turntable along a radial path. For this purpose parallel guide means for the slide are provided, which guide means comprise a sliding rod secured to the subframe. A motor unit secured to the subframe drives the slide via a pinion and a gear rack. To correct for an oblique position of the disc surface the subframe is pivotally mounted on the frame. For this purpose a shaft arranged in the center of the subframe enables the subframe to be tilted relative to the frame about a pivotal axis which coincides with the shaft. The pivotal axis extends transversely of the plane defined by the optical axis of the optical scanning unit and the axis of rotation of the turntable. For tilting the subframe about the axis a further motor unit is provided. Thus, in the known construction the position of the scanning unit is adapted to the oblique position of the optical disc by tilting the assembly comprising the subframe, the parallel guide means for the slide, and the slide itself relative to the frame.
An electro-optical device which is closely related to the prior-art device described in the foregoing is disclosed in European Patent Application 0,196,691 to which U.S. Pat. No. 4,694,442 corresponds; (herewith incorporated by reference). However, in this known device the motor unit for the slide is secured to the movable sub-frame and is coupled to the slide via a rather intricate transmission mechanism.
A disadvantage of the known electro-optical devices is the intricate tilting constructions necessary to correct the position of the optical scanning element relative to the surface of the optical disc during scanning of the disc. As will be evident from the foregoing, these tilting constructions comprise a subframe which is pivotally movable in the frame and a slide which is linearly movable in the subframe and which carries the optical scanning element. An additional disadvantage of the device disclosed in U.S. Pat. No. 4,637,004 resides in the location where the motor unit for the slide is secured to the subframe. When the motor unit is secured at this location this increases the likelihood of undesired mechanical vibrations being set up in the device, while in addition it necessitates the use of flexible electrical wiring to the motor unit.